Part III — Sizing

How do you get from a stated workload to a number of machines you would defend?

Everything so far has been preparation. This part does the arithmetic, and then looks at what it produced.

ch09 · Capacity is the chain from what an application stores to what a purchase order says, and the asymmetry in it: every term that hurts is certain, and the one term that helps is a measurement.

ch10 · Three chains, and the binding constraint is the second chain, the fact that the two disagree, and the number nobody computes: how far short you fall in the cases where the chain you dropped is the one that binds.

ch11 · Headroom and failure domains is the margin under both, and why it is a rule rather than a number. It also has the arithmetic that catches people: margins compose by multiplication, and nobody in the room multiplies them.

ch12 · The sizing model assembles all of it and produces the number the book is about. Then it asks the model what that number is worth. The answer is the reason Part IV exists.

The transition out of this part is the only one in the book that is a feeling rather than a topic: you have a number, and you cannot defend it.